Present status of micro- and polycrystalline silicon solar cells made by hot-wire chemical vapor deposition

Abstract

Considerable effort is presently put into the development of thin film microcrystalline silicon, because it has a larger long-wavelength response than amorphous silicon, while at the same time it is essentially stable. In this review, the latest achievements in this field as obtained by hot-wire chemical vapor deposition (HWCVD) technology are presented and illustrated by the performance of silicon thin film devices. Since microcrystalline silicon has an indirect band gap, the absorption coefficient is low. Nevertheless, using light trapping geometries, the required thickness can be kept below 2 μm. In spite of this small thickness long deposition times are still required and therefore the achievement of higher deposition rates is important for production. Alternatively, multijunction cells, including amorphous components made at a higher deposition rate, can lead to lower costs while improving the overall efficiency. By doubling the catalytic surface in HWCVD, we have recently achieved a deposition rate of 7 nm/s for polycrystalline silicon. As these layers tend to be more porous, a new optimum in the gas-phase reaction chemistry was found in a regime of reduced filament temperature and higher hydrogen dilution. This has led to polycrystalline n–i–p type cells made at more than twice the deposition rate while reproducing the cell efficiency. If the crystallinity is relaxed, allowing an increase of the amorphous volume fraction, microcrystalline silicon (rather than polycrystalline silicon) is obtained. In this mode, the layers more readily possess the compactness that is required to prevent post-oxidation. We present the world's first HWCVD multibandgap triple junction cell with an efficiency of 9.1% on plain stainless steel and show the future potential of this technology.